Synthesis, Opioid Receptor Binding, and Bioassay of Naltrindole

2872

J. Med. Chem. 1998, 41, 2872-2881

Synthesis, Opioid Receptor Binding, and Bioassay of Naltrindole Analogues Substituted in the Indolic Benzene Moiety Subramaniam Ananthan,*,† Cheryl A. Johnson,† Ronald L. Carter,† Sarah D. Clayton,† Kenner C. Rice,§ Heng Xu,| Peg Davis,‡ Frank Porreca,‡ and Richard B. Rothman| Organic Chemistry Department, Southern Research Institute, Birmingham, Alabama 35255, Department of Pharmacology, College of Medicine, The University of Arizona Health Sciences Center, Tucson, Arizona 85724, Laboratory of Medicinal Chemistry, Building 8, Room B1-23, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, and Clinical Psychopharmacology Section, National Institute on Drug Abuse, Addiction Research Center, Baltimore, Maryland 21224 Received February 6, 1998

A series of analogues of the δ opioid receptor antagonist naltrindole (1) possessing a phenyl, phenoxy, or benzyloxy group at the 4′-, 5′-, 6′-, or - 7′-positions (4-15) and a 2-(2-pyridinyl)ethenyl group at the 5′-position (16) on the indolic benzene ring were synthesized through Fischer indolization of naltrexone. Compounds 4-16 were evaluated for their affinities in opioid receptor binding assays in rat or guinea pig brain membranes and for their opioid antagonist and agonist activities in vitro on the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations. All of the compounds displayed δ selectivity in binding to the δ, µ, and κ opioid receptors. The binding potencies of most of the compounds at the δ, µ, and κ sites, however, were lower than that of 1. Among positional isomers, the 7′-substituted compounds in general had higher affinities than 6′-, 5′-, or 4′-substituted analogues, indicating that bulky groups are tolerated better at the 7′-position than at other positions. The affinity of the compounds were also determined at putative subtypes of the δ and κ receptors: δcx-1 (µ-like), δcx-2 (δ-like), and the κ2b site in an attempt to identify subtype selective agents. Although none were identified, the data revealed a different rank-order of potency beteween µ vs δcx-1, δcx-2 vs δ, and the κ2b vs µ, δ, and κ1. The antagonist potencies of the compounds in the MVD were in agreement with their binding affinities at the δ site in rat brain membrane. The most potent member of the series, the 7′-phenoxy compound 14, binds to the δ site with a Ki of 0.71 nM, shows >40-fold δ over µ and δ over κ binding selectivity, and exhibits δ receptor antagonist potency in the MVD with a Ke of 0.25 nM, properties which are comparable to the δ receptor affinity and antagonist potency of naltrindole (Ki ) 0.29 nM, Ke ) 0.49 nM). Interestingly, many members of the series were found to possess significant partial to full agonist activities in the MVD (6, 9, 10, 13, 16) or GPI (6, 11, 14, 15). Among the compounds studied, the highest agonist activity in the MVD was displayed by 16 (IC50 ) 220 nM), and the highest agonist activity in the GPI was displayed by 14 (IC50 ) 450 nM). The overall affinity and activity profile of compound 14 is, therefore, that of a nonpeptide ligand possessing mixed µ agonist/δ antagonist properties. Recently there has been considerable interest in such compounds possessing µ agonist/δ antagonist activities because of their potential therapeutic usefulness as analgesics with low propensity to produce tolerance and dependence side effects. The results of the present study suggest that morphinan derivatives related to 16 and 14 may provide useful leads for the development of potent nonpeptide ligands possessing δ agonist or mixed δ antagonist/µ agonist activities. The existence of three major types of opioid receptors, referred to as mu (µ), delta (δ), and kappa (κ) receptors is widely accepted and well supported by a variety of experimental data including recent cloning of µ-, δ-, and κ-opioid receptors.1-8 Among these three major classes of opioid receptors, recent research efforts have focused on the development of agonist and antagonist ligands selective for the δ opioid receptors.9-14 Various studies suggest that δ selective agonists could be potentially useful as analgesics devoid of side effects such as * To whom correspondence should be addressed. Phone: (205) 5812822. Fax: (205) 581-2726. E-mail: [email protected]. † Southern Research Institute. ‡ University of Arizona Health Sciences Center. § National Institute of Diabetes and Digestive and Kidney Diseases. | National Institute on Drug Abuse.

respiratory depression, physical dependence, and gastrointestinal effects.15 Selective antagonists of δ receptors have been shown to modulate the development of tolerance and dependence to µ agonists such as morphine,16 modulate the behavioral effects of drugs of abuse such as cocaine,17,18 and to elicit immunomodulatory effects.19,20 Thus, the δ opioid receptor system has become an attractive candidate for the development of analgesics, immunomodulatory agents, and treatment agents for drug addiction. Pharmacological studies in vitro and in vivo have suggested the existence of subtypes of the δ opioid receptors, termed δ1 and δ2.21-25 On the basis of ligand binding studies the δ receptor sites have also been distinguished as δ sites associated with a putative µ-δ

S0022-2623(98)00083-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998

Indolic Benzene Substituted Naltrindole Analogues

Journal of Medicinal Chemistry, 1998, Vol. 41, No. 15

opioid receptor complex,26 termed the δcx site, and δ sites not associated with the µ-δ opioid receptor complex, termed the δncx site.26-29 Binding, pharmacological, and molecular biological studies indicate that the cloned δ receptor is related to the pharmacologically defined δ2 receptor30,31 and likely to the δncx-2 binding site.32 The relationship between the δ subtypes defined in binding assays26-29 and the δ1 and δ2 receptors remains to be fully elucidated. Among nonpeptide opioid ligands, the indolomorphinan naltrindole (1, NTI) developed by Portoghese and co-workers is widely used as a highly selective and potent δ opioid receptor antagonist and has served as a prototype molecule for structural manipulations in the search for novel agonist and antagonist ligands possessing opioid receptor subtype selectivity.33-49 Although NTI itself does not display selectivity for the putative subtypes of the δ receptor, substituted analogues of NTI such as the 5′-isothiocyanato derivative 2 (NTII) and the 1′-benzyl derivative 3 (BNTI) have been found to possess pharmacological selectivity for the δ2 receptor.23,36,41 The high δ-opioid antagonist potency and selectivity of NTI have been ascribed to the indolic benzene moiety, which mimics a portion of the putative “address” component (Phe4) of enkephalin.45 Previous structureactivity studies on NTI analogues focusing on the indolic benzene ring have included ligands with benzannulations at the 5′,6′- and 6′,7′-positions, as well as compounds possessing substituents such as F, Br, Me, NO2, NCS, OH, OMe, OEt at the 4′-, 5′-, 6′-, or 7′-positions and basic alkylamidinomethyl groups at the 5′-position.33,34,36,37,40 Steric tolerance for bulky groups on the indolic benzene moiety of the indolomorphinans at the ligand binding sites of the opioid receptors, however, has remained largely unexplored. The present study was therefore undertaken to probe the effect of bulky aromatic group substitutions at the indolic benzene ring of NTI on the opioid receptor binding profile and intrinsic activity. The affinities of the compounds were also determined at putative subtypes of the δ and κ receptors, δcx-1 (µ-like), δcx-2 (δ-like), and the κ2b site, in an attempt to identify subtype selective agents. Herein we report the synthesis and evaluation of a series of such analogues 4-16 (Chart 1) possessing primarily a phenyl group directly attached to the indolic benzene ring of NTI or tethered to it through a one- or two-atom spacer group. The choice of the O or OCH2 linkage for tethering the phenyl group to the indolic benzene ring was primarily based on the synthetic accessibility of the needed intermediates.

Chart 1

Chemistry The target compounds 4-16 were synthesized by the Fischer indolization of naltrexone with the appropriate phenylhydrazine (Scheme 1). With the exception of 2-(benzyloxy)phenylhydrazine, all of the required phenylhydrazines carrying phenyl-, phenoxy-, and benzyloxy substituents at the ortho-, meta-, and para-positions were synthesized essentially according to literature methods starting from the corresponding anilines via diazotization and reduction.46-50 The 2-(benzyloxy)phenylhydrazine was prepared by a literature procedure involving sydnone hydrolysis.51 The Fischer indolizations were carried out using hydrogen chloride-satu-

2873

Scheme 1

rated methanol or glacial acetic acid as the reaction medium. The ortho- and para-substituted phenylhydrazines yielded the corresponding 7′-or 5′-substituted target compounds 13-15 and 7-9, respectively. As expected, the condensation of naltrexone with metasubstituted phenylhydrazines yielded predominantly the 6′-substituted isomers 10-12 along with minor amounts of the 4′-isomers 4-6. In the condensation of naltrexone with 3-(benzyloxy)phenylhydrazine, when methanolic hydrogen chloride was used as the acid catalyst the indolization proceeded with concomitant O-debenzylation to yield predominantly 6′-hydroxynaltrindole as the isolable product. This debenzylation could be avoided by using acetic acid as the reaction medium to obtain the desired 4′- and 6′-benzyloxy target compounds 6 and 12. The 5′-(2-pyridinylethenyl) compound 16 was obtained by condensation of naltrexone

2874


Ananthan et al.

Table 1. Opioid Receptor Binding Affinities of NTI Analogues in Homogenates of Rat or Guinea Pig Brain Membranes

Ki (nM) ( SEM

selectivity ratio

compd

R

δa

µb

κ1c

δcx1d

δcx2e

κ2bf

µ/δ

k1/δ

4 5 6 7 8 9 10 11 12 13 14 15 16 1

4′-C6H5 4′-OC6H5 4′-OCH2C6H5 5′-C6H5 5′-OC6H5 5′-OCH2C6H5 6′-C6H5 6′-OC6H5 6′-OCH2C6H5 7′-C6H5 7′-OC6H5 7′-OCH2C6H5 5′-(E)-CHdCH-2-Py H

52.4 ( 9.2 67.9 ( 1.9 14.8 ( 1.5 15.8 ( 1.3 17.9 ( 1.4 4.4 ( 0.51 18.0 ( 2.0 6.4 ( 1.2 6.7 ( 0.9 2.1 ( 0.26 0.71 ( 0.08 3.1 ( 0.54 3.7 ( 0.29 0.29 ( 0.06

1510 ( 106 1430 ( 126 360 ( 32 165 ( 29 292 ( 29 81 ( 6.5 1280 ( 219 235 ( 14 676 ( 69 124 ( 10 34.2 ( 9.4 159 ( 17 76 ( 9.4 50 ( 2.3

2970 ( 368 1310 ( 114 539 ( 85 354 ( 23 75.2 ( 19 82.1 ( 14 632 ( 85 236 ( 66 470 ( 52 312 ( 23 33.6 ( 2.6 258 ( 66 20.1 ( 2.4 34.1 ( 4.1

284 ( 14.3 395 ( 85.7 74.1 ( 10.2 121 ( 24.1 204 ( 50.5 14.9 ( 2.6 299 ( 46.0 89.4 ( 20.5 201 ( 58.4 51.4 ( 7.1 18.7 ( 1.4 73.3 ( 10.7 17.4 ( 2.5 12.3 ( 1.4

127 ( 13.7 114 ( 8.5 17.2 ( 4.8 24.7 ( 2.2 4.9 ( 0.5 1.7 ( 0.17 14.2 ( 3.5 4.9 ( 0.49 6.3 ( 0.5 0.46 ( 0.06 0.61 ( 0.04 2.5 ( 0.13 4.0 ( 0.59 0.24 ( 0.02

435 ( 35.3 353 ( 21.7 109 ( 10.3 186 ( 10.6 199 ( 40.6 85.8 ( 17.4 660 ( 43.6 175 ( 23.7 461 ( 43.4 89.9 ( 7.5 56.8 ( 5.1 112 ( 15.1 21.4 ( 1.2 19.9 ( 1.4

29 21 24 10 16 18 71 37 101 59 48 51 21 172

57 19 36 22 4 19 35 37 70 149 47 83 5 118

a Displacement of [3H]DADLE (1.3-2.0 nM) in rat brain membranes using 100 nM DAMGO to block binding to µ-sites. b Displacement of [3H]DAMGO (1.4-2.0 nM) in rat brain membranes. c Displacement of [3H]U69,593 (1.2-2.2 nM) in guinea pig brain membranes depleted of µ and δ binding sites by pretreatment with irreversible ligands BIT and FIT. d Displacement of [3H]DADLE (2.0 nM) in the presence of 300 nM DELT-II in rat brain membranes depleted of δncx sites by treatment with δncx-selective acylating agent, (+)-trans-SUPERFIT. eDiplacement of [3H]DADLE (2.0 nM) in the presence of 50 nM morphine using rat brain membranes depleted of δ ncx sites by pretreatment with the δncx-selective acylating agent, (+)-trans-SUPERFIT. fDisplacement of [125I]IOXY (0.01 nM) in the presence of 5 µM (-)U50,488 using guinea-pig brain membranes depleted of µ and δ binding sites by pretreatment with BIT and FIT.

with the commercially available 4-(2-pyridinylethenyl)phenylhydrazine. Results and Discussion Opioid Receptor Binding. The binding affinities of the target compounds for the µ and δ receptors were determined by inhibition of binding of [3H]DAMGO52 and [3H]DADLE53 to rat brain membranes. The affinities of the compounds for the κ receptors were determined by inhibition of binding of [3H]U69,59354 to guinea pig brain membranes. Since NTI binds with high affinity and selectivity for the morphine-insensitive δcx-2 sites over the morphine-sensitive δcx-1 sites and with moderate affinity to the κ2b site, the target compounds were also evaluated for their affinities at the δcx-1, δcx-2, and κ2b sites using previously reported methods.28,55 The affinities of the target compounds for binding at the µ, δ, κ1, δcx-1, δcx-2, and κ2b sites are listed in Table 1. All of the compounds bind with greater affinity at the δ receptor than at µ, κ1, or κ2b sites. The µ/δ binding selectivity of the target compounds are in the range of 10- to 100-fold. The overall affinity pattern for the phenyl-, phenoxy-, and benzyloxy naltrindoles at the δ receptor indicates that these bulky substituents, in general, are better tolerated at the 7′-position than at 4′-, 5′-, or 6′-positions, the δ receptor affinity generally decreasing in the following order: 7′ > 6′ = 5′ > 4′. The most potent member of the series, the 7′-phenoxy compound 14, is nearly as potent as naltrindole at the δ, µ, and κ1 sites. Among the phenyl, phenoxy, and benzyloxy substituents at the 4′-, 5′-, or 6′-positions, the benzyloxy group appears to be better tolerated at the δ binding site. It is possible that the increased conformational flexibility of the benzyloxy side chain may

allow for a better accommodation of this substituent at the δ receptor sites. The 6′-benzyloxy compound 12 shows the highest (101-fold) µ/δ selectivity ratio among this group of compounds. In general, the 6′- and 7′-substituted compounds display greater δ selectivities than the 4′- and 5′-substituted compounds. The affinities of the compounds at the δcx-2 site parallel their affinities at the δ site. While most of the compounds show equal or slightly reduced affinities at the δcx-2 site in comparison to their potency at δ site, a slight enhancement in affinities is observed for compounds 8, 9, and 13. The δcx-1 and δcx-2 affinity profiles of the target compounds are in conformity with previous studies, indicating that the ligand selectivity profiles at the δcx-1 and δcx-2 sites in general reflect the selectivity profiles at the classical µ- and δ-sites, respectively.28 The affinities of the target compounds at the κ1 site are in the same range as their affinities at the µ-site, their κ1/δ selectivity ratios ranging from 4-fold for 8 to 149-fold for 13. Highest affinity for binding at the κ1 site is displayed by the 5′-pyridinylethenyl compound 16. With the exception of 8 and 14, the affinities of all of the compounds are greater at the κ2b site than they are at the κ1 site. Of interest are the affinity profiles of the two-atom bridged 5′-benzyloxy and 5′-pyridinylethenyl compounds 12 and 16 at the δ and κ1 sites. Compared to NTI, compound 12 possesses lower affinities at both the δ and κ1 sites with 23-fold reduction at the δ-site and 14-fold reduction at the κ1 site. While the affinity of 16 is also reduced 13-fold at the δ site, relative to NTI, the affinity of 16 at the κ1 site shows 1.7-fold enhancement. Earlier studies on naltrindole analogues of the type 17 have revealed that the introduction of basic groups such as alkylamidinomethyl groups at the 5′-position leads to a remarkable change



2875

in opioid receptor selectivity from δ to κ.34 The enhanced κ receptor affinity and selectivity of 17 as well as that of ligands such as norbinaltorphimine have been attributed to favorable interaction of the basic moieties present in these ligands with an acidic residue such as Glu297 at the TM6 of the κ receptor.56,57 Therefore it appears likely that weak, favorable interactions provided by the basic nitrogen of the pyridinylethenyl substituent of 16 may be contributing to its enhanced binding affinity at the κ1 site. All of the benzene ring substituted naltrindole targets have retained the δ-selective binding property of the parent compound, albeit with moderate to significant reductions in their affinities in binding to the δ site. The inability of the benzene ring substituents to substantially modify the binding selectivity profile of NTI at the putative δ receptor subtypes suggests that the environment in the ligand binding pocket that accommodates the indolic benzenoid moiety of the indolomorphinans may be somewhat similar in these opioid receptor subtypes. As noted above, the ligand-selectivity pattern of the δcx-2 site is similar to that of the δ site, and the ligand-selectivity pattern of the δcx-1 site is similar to that of the µ site. Moreover, the ligandselectivity pattern of the κ2b site is somewhat similar to the µ and δ sites.55 To probe for differences in ligandselectivity patterns, the data was first normalized to that of the reference compound, naltrindole, by dividing the Ki values of each analogue by the Ki of naltrindole for that site to yield a “normalized Ki” value. To compare two sites, for example, µ vs δ, the following calculation was made: 1 - ([normalized Ki µ]/[normalized Ki δ]). The term [normalized Ki µ]/[normalized Ki δ] expresses the relative affinities. If two sites are identical, the expected ratio would be 1.0. Subtracting this value from 1 expresses the difference between expected and observed values. If two sites are identical, then the difference will be zero. Deviation from the expected value of 0 indicates a different ligand-selectivity profile. We made the following comparisons shown in Figure 1: µ vs δcx-1 (panel A), δ vs δcx-2 (panel B), µ vs κ2b (panel C), δ vs κ2b (panel D), and κ1 vs κ2b (panel E). These results show that compounds 7 and 8 best distinguish between µ and δcx-1, that compounds 8 and 13 best distinguish between δ and δcx-2, that compounds 7 and 14 best distinguish between µ and κ2b, that compounds 5 and 16 best distinguish between δ vs κ2b, and that compounds 4 and 6 best distinguish between κ1 and κ2b. Bioassays in Smooth Muscle Preparations. The target compounds were evaluated for antagonist and agonist activities on electrically stimulated guinea pig ileal longitudinal muscle myenteric plexus (GPI) and the mouse vas deferens (MVD) smooth muscle preparations using previously described bioassays.58 The standard agonist ligands DPDPE and PL 017 possessing pharmacological selectivity for µ and δ opioid receptor, respectively, were employed for the evaluation of antagonist potency. The compounds were incubated with the preparations for 30 min and washed with buffer prior to testing with the standard agonists. The antagonist potency is expressed as IC50 ratios and as Ke values. The IC50 ratio represents the agonist potency in the presence of the test compound divided by the

control IC50 in the same preparation. The Ke values were calculated from the expression Ke ) [antagonist]/ (IC50 ratio -1). The agonist activities of the compounds were evaluated in the MVD and GPI and are expressed as percent of maximal response at 1 µM or, for more potent ligands, as IC50 values. The opioid antagonist and agonist potencies of the target compounds and the reference ligand NTI are listed in Table 2. All of the compounds, with the exception of 6 and 16, exhibited antagonist activity against the agonist DPDPE in the MVD. The δ-antagonist potencies of the compounds in the MVD correspond fairly closely with their δ-site affinities determined in the ligand binding assays. Indeed a comparison of the Ki and Ke values afforded a highly significant relationship (r2 ) 0.87, P < 0.000006). Among the positional isomers, high antagonist potencies in the MVD are displayed by the 7′-substituted compounds 13-15. The most potent member of this series, the 7′-phenoxy compound 14 (Ke ) 0.25 nM), is twice as potent as NTI in its antagonist activity at the δ receptor. The 5′- and 6′-substituted analogues 7-12 display diminished antagonist potencies with Ke values in the range of 4-20 nM. These results are in harmony with the observations of Portoghese and co-workers who found a similar reduction in δ antagonist potencies in compounds bearing substituents at the 5′- or 6′-position of naltrindole.34 In comparison to their antagonist potencies in the MVD, all of the compounds are much weaker as antagonists in the GPI and, as a consequence, these analogues possess 10 to >170-fold δ over µ antagonist selectivities. Most of the target compounds displayed partial agonist activity at 1 µM concentration in the MVD with inhibition of maximal response in the range of 11-36%. Two compounds that showed higher agonist responses were compounds 6 (70% maximal response) and 16 (79% maximal response, IC50 ) 220 nM). Interestingly, several compounds displayed agonist activity in the GPI preparations. Compounds exhibiting >50% inhibition of maximal response in the GPI include 11, 14 and 15 as well as 6 and 16. Among the four compounds (11, 14, 15, 16) for which agonist IC50 values were determined, the most potent was found to be the 7′-phenoxy compound 14 (IC50 ) 450 nM). Regarding the MVD/ GPI agonist selectivity, 11 is relatively nonselective, 16 is δ-selective, while 14 and 15 are µ-selective. The agonist effects of 14 and 16 in the GPI and the MVD were antagonized by naloxone. In the presence of 1 µM of naloxone, the agonist dose-response curve of 14 was shifted to higher concentration by a factor of 5.1 in the GPI and 2.8 in the MVD. Similarly, in the presence of naloxone (1µM), the agonist IC50 values of 16 were increased 2.5-fold in the GPI and 9.1-fold in the MVD. Further work is needed to determine whether the observed agonist activities of 14 in GPI and 16 in MVD smooth muscle preparations are indeed mediated through the µ- and δ- receptors, respectively, and whether these activities correlate with their modest binding potencies at the µ and δ sites in the rat brain membranes. Studies in mice and rats have demonstrated that δ opioid antagonists such as NTI and TIPP can prevent the development of tolerance and dependence to µ agonists such as morphine.15,59 On the basis of these observations it has been suggested that the development

2876


Ananthan et al.

Figure 1. Difference scores for compounds 4-16 for binding at µ vs δcx-1 (panel A), δ vs δcx-2 (panel B), µ vs κ2b (panel C), δ vs κ2b (panel D) and κ1 vs κ2b sites (panel E). Difference scores were calculated using the formula 1 - [(Ki of compound at the first site/Ki of naltrindole at the first site)]/[(Ki of compound at the second site/Ki of naltrindole at the second site)]. As described in the text, the difference scores reflect the magnitude of differences in binding affinities at two binding sites relative to that of naltrindole (1).

of compounds possessing mixed µ agonist/δ antagonist properties may have considerable therapeutic potential as analgesic drugs for treating chronic pain.16,59-61 Among the compounds examined in the present study, the 7′-phenoxy compound 14 is therefore of particular interest in that it displays the properties of a mixed δ antagonist/µ agonist with δ antagonist potency equal to that of NTI and µ agonist potency nearly 1/4 that of morphine in the GPI bioassay.62

Summary and Conclusions We have investigated the opioid receptor affinity and activity profile of a series of naltrindole analogues possessing phenyl, phenoxy, and benzyloxy substituents in the indolic benzene moiety. The results of the present study indicate that the introduction of bulky aromatic substituents such as phenyl, phenoxy, and benzyloxy groups at the 4′-, 5′-, 6′-, or 7′-position on the indolomorphinan framework of naltrindole affects the affinity



2877

Table 2. Opioid Antagonist and Agonist Potencies of NTI Analogues in the MVD and GPI Preparations antagonist activity DPDPE compd

IC50 ratio

4 5 6 7 8 9 10 11 12 13 14 15 16 1

16 ( 7 61 ( 16 e 110 ( 0.3 93 ( 7 210 ( 29 49 ( 7 220 ( 3 260 ( 50 560 ( 90 4000 ( 200 610 ( 110 e 2000 ( 400

(δ)a

PL017 (µ)b

Ke (nM)c 66 17 9.4 11 4.8 21 4.6 3.9 1.8 0.25 1.7 0.49

agonist activity

IC50 ratio

Ke (nM)c

Ke selectivity ratio µ/δ

MVD IC50 (nM) or % max respd

GPI IC50 (nM) or % max respd

2.4 ( 0.2 2.7 ( 0.1 e 10 ( 3 2.3 ( 0.5 2.8 ( 0.1 f e f 4.2 ( 1 e e e 24 ( 2

710 590

11 35

110 770 560

12 70 117

22% 27% 70% 25% 11% 35% 36% 3000 ( 1000 25% 35% 4100 ( 2100 9500 ( 5200 220 ( 70 16%

0% 28% 86% 0% 8% 24% 2% 2300 ( 200 0% 37% 450 ( 40 1700 ( 500 2600 ( 200 18%

310

172

43

88

IC50 selectivity ratio GPI(µ)/MVD(δ)

0.8 0.11 0.18 12

DPDPE in the MVD preparation. b PL-017 in the GPI preparation. c Ke (nM) ) [antagonist]/(IC50 ratio - 1), where the IC50 ratio is the IC50 of the agonist in the presence of antagonist divided by the control IC50 in the same preparation (n g 3). d Partial agonist activity is expressed as the percentage inhibition of contraction at a concentration of 1 µM. eThe agonist effects precluded the determination of antagonist effects. fIC50 ratio was not statistically different from 1. a

as well as the intrinsic activity to varying degrees depending upon the nature of the substituent and the substitution pattern. Among the various indolic benzene ring positions, bulky substitutions are better tolerated at the 7′-position than at others. The results also demonstrate moderate ligand-selectivity differences between the µ-like sites (µ vs δcx-1), the δ-like sites (δ vs δcx-2), and the κ2b sites and µ, δ and κ1 sites, supporting the hypothesis that these are distinct binding sites. While most of the substituted naltrindole analogues retain the δ antagonist property of the parent compound, some of the analogues display moderate to significant opioid agonist activity in the MVD (6, 9, 10, 13, 16) and GPI (6, 11, 14, 15). Of special interest is the finding that compound 14 possesses affinity and activity profile of a nonpeptide opioid ligand with mixed µ agonist/δ antagonist properties, since such compounds may have considerable potential for development as analgesics with a low propensity to produce tolerance and dependence side effects. Further structural manipulations of the indolomorphinan framework may, therefore, lead to more potent nonpeptide ligands possessing δ antagonist or δ agonist as well as mixed µ agonist/δ antagonist properties. Experimental Section General Methods. Melting points were determined in open capillary tubes with a Mel-Temp melting point apparatus and are uncorrected. 1H NMR spectra were recorded on a Nicolet 300NB spectrometer operating at 300.635 MHz. Chemical shifts are expressed in parts per million downfield from tetramethylsilane. Spectral assignments were supported by proton decoupling. Mass spectra were recorded on a Varian MAT 311A double-focusing mass spectrometer in the fast atom bombardment (FAB) mode. Elemental analyses were performed by Atlantic Microlab, Inc. (Atlanta, GA) or the Molecular Spectroscopy Section of Southern Research Institute. Analytical results indicated by elemental symbols were within (0.4% of the theoretical values. Thin-layer chromatography (TLC) was performed on Analtech silica gel GF 0.25 mm plates. Preparative TLC was performed on Analtech silica gel GF 2.0 mm plates. Flash column chromatography was performed with E. Merck silica gel 60 (230-400 mesh). All organic extracts were dried over anhydrous Na2SO4 and concentrated to dryness on a rotary evaporator under reduced

pressure. All reagents were obtained from Aldrich Chemical Co. Naltrexone hydrochloride was obtained from Mallinckrodt. 4′-Hydrazino-2-stilbazole dihydrochloride was purchased from TCI America. Chemistry. General Procedures. Fischer Indole Synthesis. Method A. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-4′-phenylindolo[2′,3′:6,7]morphinan (4) and 17-(Cyclopropylmethyl)-6,7-didehydro4,5r-epoxy-13,14-dihydroxy-6′-phenylindolo[2′,3′:6,7]morphinan (10). A suspension of naltrexone hydrochloride (1.51 g, 4.0 mmol) and 3-biphenylylhydrazine hydrochloride (1.32 g, 6.0 mmol) in glacial acetic acid (40 mL) was refluxed for 5 h with stirring in an oil bath at 120 °C. After cooling, the reaction mixture was concentrated under reduced pressure, the residue was suspended in water (20 mL), and the pH of the mixture was adjusted to 7.0 with saturated aqueous NaHCO3. The solid obtained was collected by filtration and dried. The crude product was chromatographed on a column of silica gel using CHCl3-MeOH (9:1) as the eluent to obtain a major less polar fraction containing the 6′-isomer and a minor fraction containing the 4′-isomer. Further purification of the polar fraction by preparative TLC (CHCl3-MeOH 12: 1), followed by crystallization from MeOH/H2O yielded 0.057 g (3%) of 4: mp >186 °C dec; TLC, Rf 0.47 (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.08 and 0.43 (2m, 4H, cyclopropyl CH2CH2), 0.82 (m, 1H, cyclopropyl CH), 1.54 (d, 1H, C-15 H), 1.93-1.98 (m, 1H, C-8 H), 2.02-2.40 (m, 5H, C-8 H, C-15 H, C-16 H, NCH2-cyclopropyl), 2.40-2.58 (m, 1H, C-10 H), 2.66 (m, 1H, C-16 H), 2.84 (d, 1H, C-9 H), 2.99 (d, 1H, C-10 H), 4.56 (bs, 1H, C-14 OH), 5.48 (s, 1H, C-5 H), 6.45-6.57 (m, 2H, C-1 H, C-2 H), 6.77 (dd, 1H, C-7′ H), 7.12 (t, 1H, C-6′ H), 7.24 and 7.32-7.44 (d and m, 6H, C-5′ H, C-2′′ H, C-3′′ H, C-4′′ H, C-5′′ H, C-6′′ H), 8.97 (s, 1H, C-3 OH), 11.38 (s, 1H, NH); MS m/z 491 (MH)+. Anal. (C32H30N2O3‚0.5H2O) C, H, N. Recrystallization of the less polar product from EtOAc/cyclohexane yielded 0.355 g (18%) of 10: mp >198 °C dec; TLC, Rf 0.52 (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.17 and 0.53 (2m, 4H, cyclopropyl CH2CH2), 0.92 (m, 1H, cyclopropyl CH), 1.61 (d, 1H, C-15 H), 2.18 (m, 1H, C-16 H), 2.27-2.53 (m, 4H, C-8 H, C-15 H, NCH2-cyclopropyl), 2.66-2.82 (m, 3H, C-8 H, C-10 H, C-16 H), 3.09 (d, 1H, C-10 H), 3.30 (d, 1H, C-9 H), 4.77 (s, 1H, C-14 OH), 5.54 (s, 1H, C-5 H), 6.45-6.54 (m, 2H, C-1 H, C-2 H), 7.28 (m, 2H, C-4′ H, C-5′ H), 7.44 (t, 3H, C-3′′ H, C-4′′ H, C-5′′ H), 7.56 (s, 1H, C-7′ H), 7.65 (d, 2H, C-2′′ H, C-6′′ H), 8.94 (s, 1H, C-3 OH), 11.24 (s, 1H, NH); MS m/z 491 (MH)+. Anal. (C32H30N2O3‚0.2CH3CO2C2H5) C, H, N. Fischer Indole Synthesis. Method B. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-4′phenoxyindolo[2′,3′:6,7]morphinan (5) and 17-(Cyclo-

2878


Ananthan et al.

propylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy6′-phenoxyindolo[2′,3′:6,7]morphinan (11). A suspension of naltrexone hydrochloride (1.89 g, 5.0 mmol) and 3-phenoxyphenylhydrazine hydrochloride (1.98 g, 7.5 mmol) in methanolic HCl (100 mL) was heated with stirring under reflux at 90° C under N2 for 5 h. The reaction mixture was concentrated and diluted with ice water (200 mL), and the pH was adjusted to 7.0 with saturated aq. NaHCO3. The solid obtained was collected by filtration and dried. Column chromatography of the crude product on silica gel using hexane/EtOAc/Et3N, 1:1: 0.04 as the eluent yielded a major fraction containing the less polar 6′-isomer and a minor fraction containing the 4′-isomer. Preparative TLC (CHCl3-MeOH 9.5:0.5) of the more polar fraction gave 0.024 g (0.9%) of 5: mp >154° C dec; TLC, Rf 0.45 (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.13 and 0.48 (2m, 4H, cyclopropyl CH2CH2), 0.87 (m, 1H, cyclopropyl CH), 1.57 (d, 1H, C-15 H), 2.13 (m, 1H, C-16 H), 2.24-2.90 (m, 7H, C-8 H2, C-10 H, NCH2-cyclopropyl, C-15 H, C-16 H), 3.00 (d, 1H, C-10 H), 3.14 (s, 1H, C-9 H), 4.71 (bs, 1H, C-14 OH), 5.49 (s, 1H, C-5 H), 6.30 (d, 1H, C-5′ H), 6.42-6.55 (m, 2H, C-1 H, C-2 H), 6.97 (m, 3H, C-6′ H, C-2′′ H, C-6′′ H), 7.09 (m, 2H, C-7′ H, C-4′′ H), 7.32 (m, 2H, C-3′′ H, C-5′′ H), 8.98 (s, 1H, C-3 OH), 11.35 (s, 1H, NH); MS m/z 507 (MH)+. Anal. (C32H30N2O4‚ 0.25 H2O) C, H, N Recrystallization of the less polar product from EtOAc/ cyclohexane yielded 622 mg (25%) of 11: mp >154 °C dec; TLC, Rf 0.59, CHCl3/MeOH 9:1); 1H NMR (DMSO-d6) δ 0.15 and 0.50 (2m, 4H, cyclopropyl CH2CH2), 0.9 (m, 1H, cyclopropyl CH), 1.58 (d, 1H, C-15 H), 2.15 (m, 1H, C-16 H), 2.24-2.52 (m, 4H, C-8 H, C-15 H, NCH2-cyclopropyl), 2.64-2.80 (m, 3H, C-8 H, C-10 H, C-16 H), 3.05 (d, 1H, C-10 H), 3.28 (d, 1H, C-9 H), 4.75 (s, 1H, C-14 OH), 5.55 (s, 1H, C-5 H), 6.45-6.55 (m, 2H, C-1 H, C-2 H), 6.70 (dd, 1H, C-5′ H), 6.93 (m, 3H, C-7′ H, C-2′′ H, C-6′′ H), 7.05 (m, 1H, C-4′′ H), 7.34 (m, 3H, C-4′ H, C-3′′ H, C-5′′ H), 8.96 (s, 1H, C-3 OH), 11.12 (s, 1H, NH); MS m/z 507 (MH)+. Anal. (C32H30N2O4‚0.5H2O) C, H, N. 4′-(Benzyloxy)-17-(cyclopropylmethyl)-6,7-didehydro4,5r-epoxy-13,14-dihydroxyindolo[2′,3′:6,7]morphinan (6) and 6′-(Benzyloxy)-17-(cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxyindolo[2′,3′:6,7]morphinan (12). From naltrexone hydrochloride and 3-(benzyloxy)phenylhydrazine hydrochloride using method A. 4′-Isomer 6: yield 0.7%, mp >146 °C dec; TLC, Rf 0.42, CHCl3-MeOH 9:1); 1H NMR (DMSO-d ) δ 0.15 and 0.50 (2m, 4H, cyclopropyl 6 CH2CH2), 0.90 (m, 1H, cyclopropyl CH), 1.57 (d, 1H, C-15 H), 2.15 (m, 1H, C-15 H), 2.30 (m, 1H, C-16 H), 2.39 (d, 2H, NCH2cyclopropyl), 2.58 (m, 1H, C-8H), 2.70 (m, 2H, C-10 H, C-16 H), 3.08 (m, 2H, C-8 H, C-10 H), 3.18 (m, 1H, C-9 H), 4.71 (bs, 1H, C-14 OH), 5.15 (dd, 2H, OCH2-phenyl), 5.47 (s, 1H, C-5 H), 6.42-6.54 (m, 3H, C-1 H, C-2 H, C-5′ H), 6.92 (m, 2H, C-6′ H, C-7′ H), 7.30 (m, 1H, C-4′′ H), 7.37 (m, 2H, C-3′′ H, C-5′′ H), 7.43 (m, 2H, C-2′′ H, C-6′′ H),8.94(s,1H, C-3 OH), 11.13 (s, 1H, NH); MS m/z 521(MH)+. Anal. (C33H32N2O4‚0.25CH3CO2C2H5) C, H, N. 6′-Isomer 12: yield 33%, mp >136 °C dec; TLC, Rf 0.58, (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.15 and 0.52 (2m, 4H, cyclopropyl CH2CH2), 0.89 (m, 1H, cyclopropyl CH), 1.56 (d, 1H, C-15 H), 2.16 (m, 1H, C-16 H), 2.24-2.52 (m, 4H, C-8 H, C-15 H, NCH2 cyclopropyl), 2.62-2.79 (m, 3H, C-8 H, C-10 H, C-16 H), 3.05 (d, 1H, C-10 H), 3.26 (d, 1H, C-9 H), 4.73 (s, 1H, C-14 OH), 5.10 (s, 2H, OCH2 phenyl), 5.46 (s, 1H, C-5 H), 6.43-6.53 (m, 2H, C-1 H, C-2 H), 6.68 (dd, 1H, C-5′ H), 6.88 (d, 1H, C-7′ H), 7.24 (d, 1H, C-4′ H), 7.28-7.48 (m, 5H, C-2′′ H, C-3′′ H, C-4′′ H, C-5′′ H, C-6′′ H), 8.92 (s, 1H, C-3 OH), 10.95 (s, 1H, NH); MS m/z 521 (MH)+. Anal. (C33H32N2O4‚0.6H2O) C, H, N. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-5′-phenylindolo[2′,3′:6,7]morphinan (7). This compound was prepared by condensing naltrexone hydrochloride with 4-biphenylylhydrazine hydrochloride using method B. Yield 29%, mp >164 °C dec; TLC, Rf 0.40 (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.16 and 0.52 (2m, 4H, cyclopropyl CH2CH2), 0.90 (m, 1H, cyclopropyl CH), 1.60 (d, 1H, C-15 H), 2.17 (m, 1H, C-16 H), 2.27-2.54 (m, 4H, C-8 H, C-15 H, NCH2-

cyclopropyl), 2.66-2.84 (m, 3H, C-8 H, C-10 H, C-16 H), 3.08 (d, 1H, C-10 H), 3.3 (d, 1H, C-9 H), 4.76 (s, 1H, C-14 OH), 5.53 (s, 1H, C-5 H), 6.45-6.55 (m, 2H, C-1 H, C-2 H), 7.26 (m, 1H, C-4′′ H), 7.40 (m, 4H, C-4′ H, C-7′ H, C-3′′ H, C-5′′ H), 7.62 (m, 3H, C-6′ H, C-2′′ H, C-6′′ H), 8.96 (s, 1H, C-3 OH), 11.22 (s, 1H, NH); MS m/z 491 (MH)+. Anal. (C32H30N2O3‚ 1.2H2O) C, H, N. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-5′-phenoxyindolo[2′,3′:6,7]morphinan (8). This compound was prepared by condensing naltrexone hydrochloride with 4-phenoxyphenylhydrazine hydrochloride using method A. Yield 18%, mp (HCl) >270 °C dec; TLC, Rf 0.50 (CHCl3-MeOH 9:1); 1H NMR (DMSO-d6) δ 0.38-0.55 and 0.57-0.78 (2m, 4H, cyclopropyl CH2CH2), 1.08 (m, 1H, cyclopropyl CH), 1.82 (d, 1H, C-15 H), 2.44-2.79 (m, 3H, C-8 H, C-15 H, C-16 H), 2.83-3.00 (m, 2H, C-8 H, NCH-cyclopropyl), 3.06-3.48 (m, 4H, C-10 H2, C-16 H, NCH-cyclopropyl), 4.06 (d, 1H, C-9 H), 5.69 (s, 1H, C-5 H), 6.36 (s, 1H, C-14 OH), 6.556.69 (m, 2H, C-1 H, C-2 H), 6.88 (d, 3H, C-6′ H, C-2′′ H, C-6′′ H), 6.94-7.06 (m, 2H, C-4′ H, C-4′′ H), 7.30 (t, 2H, C-3′′ H, C-5′′ H). 7.39 (d, 1H, C-7′ H), 8.94 (bs 1H, H+), 9.25 (s, 1H, C-3 OH), 11.41 (s, 1H, NH); MS m/z 507 (MH)+. Anal. (C32H30N2O4‚HCl‚0.5H2O) C, H, N. 5′-(Benzyloxy)-17-(cyclopropylmethyl)-6,7-didehydro4,5r-epoxy-13,14-dihydroxyindolo[2′,3′:6,7]morphinan (9). This compound was prepared by condensing naltrexone hydrochloride with 4-benzyloxyphenylhydrazine hydrochloride using Method A. Yield 46%, mp (HCl) >360 °C dec; TLC, Rf 0.60 (CHCl3-MeOH-NH4OH 9:1:0.5); 1H NMR (DMSO-d6) δ 0.38-0.56 and 0.58-0.79 (2 m, 4H, cyclopropyl CH2CH2), 1.10 (m, 1H, cyclopropyl CH), 1.80 (d, 1H, C-15 H), 2.44-2.78 (m, 3H, C-8 H, C-15 H, C-16 H), 2.84-3.00 (m, 2H, C-8 H, NCHcyclopropyl), 3.06-3.49 (m, 4H, C-10 H2, C-16 H, NCHcyclopropyl), 4.08 (d, 1H, C-9 H), 5.04 (s, 2H, OCH2-phenyl), 5.65 (s, 1H, C-5 H), 6.36 (s, 1H, C-14 OH), 6.55-6.66 (m, 2H, C-1 H, C-2 H), 6.84 (dd, 1H, C-6′ H), 6.90 (d, 1H, C-4′ H), 7.227.45 (m, 6H, C-7′ H, C-2′′ H, C-3′′ H, C-4′′ H, C-5′′ H, C-6′′ H), 8.94 (bs, 1H, H+), 9.22 (s, 1H, C-3 OH), 11.17 (s, 1H, NH); MS m/z 521 (MH)+. Anal. (C33H32N2O4‚HCl‚0.25H2O) C, H, N. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-7′-phenylindolo[2′,3′:6,7]morphinan (13). This compound was prepared by condensing naltrexone hydrochloride with 2-biphenylylhydrazine hydrochloride using method B. Yield 38%, mp >160 °C dec; TLC, Rf 0.44 (CHCl3MeOH 9:1); 1H NMR (DMSO-d6) δ 0.15 and 0.52 (2m, 4H, cyclopropyl CH2CH2), 0.90 (m, 1H, cyclopropyl CH), 1.59 (d, 1H, C-15 H), 2.18 (m, 1H, C-16 H), 2.25-2.54 (m, 4H, C-8 H, C-15 H, NCH2-cyclopropyl), 2.64-2.8 (m, 3H, C-8 H, C-10 H, C-16 H), 3.08 (d, 1H, C-10 H), 3.30 (d, 1H, C-9 H), 4.75 (s, 1H, C-14 OH), 5.49 (s, 1H, C-5 H), 6.45-6.55 (m, 2H, C-1 H, C-2 H), 7.08 (m, 2H, C-4′ H, C-5′ H), 7.40 (m, 2H, C-6′ H, C-4′′ H), 7.55 (t, 2H, C-3′′ H, C-5′′ H), 7.64 (d, 2H, C-2′′ H, C-6′′ H), 8.92 (s, 1H, C-3 OH), 10.99 (s, 1H, NH); MS m/z 491 (MH)+. Anal. (C32H30N2O3‚0.5CH3CO2C2H5) C, H, N. 17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy-13,14-dihydroxy-7′-phenoxyindolo[2′,3′:6,7]morphinan (14). This compound was prepared by condensing naltrexone hydrochloride with 2-phenoxyphenylhydrazine hydrochloride using method B. Yield 35%, mp (HCl) >240 °C dec; TLC, Rf 0.40 (CHCl3-MeOH 9.5:0.5); 1H NMR (DMSO-d6) δ 0.40-0.56, 0.64, and 0.74 (3m, 4H, cyclopropyl CH2CH2), 1.11 (m, 1H, cyclopropyl CH), 1.82 (d, 1H, C-15 H), 2.52-2.80 (m, 3H, C-8 H, C-15 H, C-16 H), 2.90-3.04 (m, 2H, C-8 H, NCH-cyclopropyl), 3.06-3.52 (m, 4H, C-10 H2, C-16 H, NCH-cyclopropyl), 4.12 (d, 1H, C-9 H), 5.62 (s, 1H, C-5 H), 6.22 (s, 1H, C-14 OH), 6.576.71 (m, 3H, C-1 H, C-2 H, C-6′ H), 6.91-7.03 (m, 3H, C-2′′ H, C-6′′ H, C-5′ H), 7.08-7.20 (m, 2H, C-4′ H, C-4′′ H), 7.37 (m, 2H, C-3′′ H, C-5′′ H), 8.96 (bs, 1H, H+), 9.23 (s, 1H, C-3 OH), 11.61 (s, 1H, NH); MS m/z 507 (MH)+. Anal. (C32H30N2O4‚ HCl‚C2H5OH) C, H, N. 7′-(Benzyloxy)-17-(cyclopropylmethyl)-6,7-didehydro4,5r-epoxy-13,14-dihydroxyindolo[2′,3′:6,7]morphinan (15). This compound was prepared by condensing naltrexone hydrochloride with 2-(benzyloxy)phenylhydrazine hydrochloride



2879

using method A. Yield 30%, mp >163 °C dec; TLC, Rf 0.37 (CHCl3-MeOH 9.5:0.5); 1H NMR (DMSO-d6) δ 0.15 and 0.52 (2m, 4H, cyclopropyl CH2CH2), 0.89 (m, 1H, cyclopropyl CH), 1.59 (d, 1H, C-15 H), 2.08-2.22 (m, 1H, C-16 H), 2.24-2.55 (m, 4H, C-8 H, C-15 H, NCH2-cyclopropyl), 2.61-2.80 (m, 3H, C-8 H, C-10 H, C-16 H), 3.05 (d, 1H, C-10 H), 3.25 (d, 1H, C-9 H), 4.72 (s, 1H, C-14 OH), 5.24 (dd, 2H, OCH2-phenyl), 5.46 (s, 1H, C-5 H), 6.44-6.54 (m, 2H, C-1 H, C-2 H), 6.72 (d, 1H, C-6′ H), 6.83 (t, 1H, C-5′ H), 6.94 (d, 1H, C-4′ H), 7.30-7.45 (m, 3H, C-3′′ H, C-4′′ H, C-5′′ H), 7.59 (m, 2H, C-2′′ H, C-6′′ H), 8.93 (s, 1H, C-3 OH), 11.26 (s, 1H, NH); MS m/z 521 (MH)+. Anal. (C33H32N2O4‚0.5H2O) C, H, N. trans-17-(Cyclopropylmethyl)-6,7-didehydro-4,5r-epoxy13,14-dihydroxy-5′-[2-(2-pyridinyl)ethenyl]indolo[2′,3′: 6,7]morphinan (16). This compound was prepared by condensing naltrexone hydrochloride with 4′-hydrazino-2-stilbazole dihydrochloride using method A. Yield 44%, mp (HCl) >198 °C dec; TLC, Rf 0.48 (CHCl3-MeOH 9:1); 1H NMR (DMSOd6) δ 0.16 and 0.52 (2m, 4H, cyclopropyl CH2CH2), 0.90 (m, 1H, cyclopropyl CH), 1.59 (d, 1H, C-15 H), 2.16 (m, 1H, C-16 H), 2.25-2.53 (m, 4H, C-8 H, C-15 H, NCH2-cyclopropyl), 2.65-2.82 (m, 3H, C-8 H, C-10 H, C-16 H), 3.08 (d, 1H, C-10 H), 3.30 (d, 1H, C-9 H), 4.76 (s, 1H, C-14 OH), 5.51 (s, 1H, C-5 H), 6.45-6.55 (m, 2H, C-1 H, C-2 H), 7.15 (d, J ) 16 Hz, 1H, C-5′′ H, CHdCH-py), 7.18 (t, 1H, C-5′′ H), 7.34 (d, 1H, C-7′ H), 7.46 (d, 2H, C-6′ H, C-3′′ H) 7.62 (s, 1H, C-4′ H), 7.70 (d, J ) 16 Hz, 1H, CHdCH py), 7.73 (dt, 1H, C-4′′ H), 8.53 (d, 1H, C-6′′ H), 8.94 (s, 1H, C-3 OH), 11.28 (s, 1H, NH); MS m/z 518 (MH)+. Anal. (C33H31N3O3‚HCl‚0.4H2O) C, H, N. Biological Assays. Radioligand Binding Assays for µ, δ and K Receptors. Mu binding sites were labeled using [3H]DAMGO (1-3 nM) and rat brain membranes as previously described.52 Briefly, incubations proceeded for 4 h at 25 °C in 50 mM Tris-HCl, pH 7.4, along with a protease inhibitor cocktail (PIC). The nonspecific binding was determined using 20 µM of levallorphan. Delta binding sites were labeled using [3H]DADLE (1.7-2.9 nM) and rat brain membranes as previously described.53 Incubations proceeded for 3-4 h at 25 °C in 10 mM Tris-HCl, pH 7.4, containing 100 mM choline chloride, 3 mM MnCl2, and 100 nM DAMGO to block binding to µ sites, and PIC. Nonspecific binding was determined using 20 µM levallorphan. Kappa1 binding sites were labeled using [3H]U69,593 (3.4-4.6 nM) and guinea pig brain membranes depleted of µ and δ binding sites by pretreatment with irreversible ligands BIT and FIT as previously described,54 except that the incubation temperature was 25 °C. Incubations proceeded for 4-6 h at 25 °C in 50 mM Tris-HCl, pH 7.4, containing PIC and 1 µg/mL captopril. Nonspecific binding was determined using 1 µM U69,593. Delta-cx1 binding sites were labeled using [3H]DADLE (2.0 nM) in the presence of 300 nM DELT-II in rat brain membranes depleted of δncx sites by pretreatment with the δncx-selective acylating agent (+)-trans-SUPERFIT as previously described.28 Deltacx2 binding sites were labeled using [3H]DADLE (2.0 nM) in the presence of 50 nM morphine using rat brain membranes depleted of δncx sites by pretreatment with the δncx-selective acylating agent, (+)-trans-SUPERFIT as previously described.28 For δcx1 and δcx2 binding, incubations proceeded for 4-6 h at 25 °C in 10 mM Tris-HCl, pH 7.4, containing 100 mM NaCl, 3 mM MnCl2, 2 µM GTP, 5 mM 2-mercaptoethanol, and PIC. Kappa2b sites were labeled with [125I] IOXY (0.01 nM) in the presence of 5 µM (-)U50,488 using guinea-pig brain membranes depleted of µ and δ binding sites by pretreatment with BIT and FIT as previously described.55 Briefly, incubations proceeded for 4-6 h at 4 °C in 50 mM Tris-HCl, pH 7.4, containing 10 mM NaCl, PIC and 1 µg/mL of captopril. Each 3H ligand was displaced by 8-10 concentrations of test drug, two times. All drug dilutions were done in 10 mM TrisHCl, pH 7.4, containing 1 mg/mL bovine serum albumin. Compounds were prepared as 1 mM solution with 10 mM Tris buffer (pH 7.4) containing 10% DMSO before drug dilution. The IC50 and slope factor (N) were obtained by using the

program MLAB-PC (Civilized Software, Bethesda, MD). Ki values were calculated according to the equation Ki ) IC50/(1 + [L]/Kd). GPI and MVD Bioassays.62 Electrically induced smooth muscle contractions of mouse vas deferens and strips of guinea pig ileum longitudinal muscle myenteric plexus were used. Tissues came from male ICR mice weighing 25-40 g and male Hartley guinea pigs weighing 250-500 g. The tissues were tied to gold chain with suture silk, suspended in 20 mL baths containing 37 °C oxygenated (95% O2, 5% CO2) Krebs bicarbonate solution (magnesium free for the MVD), and allowed to equilibrate for 15 min. The tissues were then stretched to optimal length previously determined to be 1 g tension (0.5 g for MVD) and allowed to equilibrate for 15 min. The tissues were stimulated transmurally between platinum wire electrodes at 0.1 Hz, 0.4-ms pulses (2-ms pulses for MVD), and supramaximal voltage. An initial dose-response curve of DPDPE or PL-017 was constructed at the start of each assay to establish tissue effects, allowing each tissue to be used as its own control. Tissues not producing typical results were not used. Experimental compounds were added to the baths in 14-60 µL volumes. Succeeding doses of agonist were added cumulatively to the bath at 3 min intervals to produce a concentration-response curve. The tissues were then washed extensively with fresh buffer until the original contraction height was reestablished. Agonist effects of the compounds at 1 µM were measured as percent inhibition of contraction height 10 min after addition to the bath. Antagonist effects to DPDPE and PL-017 were assayed after incubation of the tissues with 1 µM concentration of the compound in the bath for 30 min. The tissues were then washed with fresh buffer for 30 min, and the agonist dose-response curve was repeated. Rightward shifts in the dose-response curves were calculated by dividing the antagonized dose-response curve IC50 value by the unantagonized IC50 value. IC50 values represent the mean of two to four tissues. IC50 estimates and their associated standard errors were determined by using a computerized nonlinear least-squares method.63

Acknowledgment. This investigation was supported by the National Institute on Drug Abuse, Grant No. DA08883. We thank Dr. James M. Riordan, Mr. Mark D. Richardson, and Ms. Joan C. Bearden for analytical and spectral data. We acknowledge the expert technical assistance of Ms. Karen McCullough in conducting the binding assays. We are grateful to Dr. John A. Secrist III and Dr. John A. Montgomery for their encouragement, valuable comments, and discussions during the course of this work. References (1) Simon, E. J.; Gioannini, T. L. Opioid Receptor Multiplicity: Isolation, Purification and Chemical Characterization of Binding Sites. In Handbook of Experimental Pharmacology, Volume 104, Opioids I; Herz, A., Akil, H., Simon, E. J., Eds; SpringerVerlag: Berlin, 1993; pp 3-26. (2) Dhawan, B. N.; Cesselin, F.; Raghubir, R.; Reisine, T.; Bradley, P. B.; Portoghese, P. S.; Hamon, M. International Union of Pharmacology. XII. Classification of Opioid Receptors. Pharmacol. Rev. 1996, 48, 567-592. (3) Chen, Y.; Mestek, A.; Liu, J.; Hurley, J. A.; Yu, L. Molecular Cloning and Functional Expression of a µ-Opioid Receptor from Rat Brain. Mol. Pharmacol. 1993, 44, 8-12. (4) Wang, J.-B.; Imai, Y.; Eppler, C. M.; Gregor, P.; Spivak, C. E.; Uhl, G. R. µ-Opiate Receptor: cDNA Cloning and Expression. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 10230-10234. (5) Evans, C. J.; Keith, D. E., Jr.; Morrison, H.; Magendzo, K.; Edwards, R. H. Cloning of a Delta Opioid Receptor by Functional Expression. Science 1992, 258, 1952-1955. (6) Kieffer, B. L.; Befort, K.; Gaveriaux-Ruff, C.; Hirth, C. G. The δ-Opioid Receptor: Isolation of a cDNA by Expression Cloning and Pharmacological Characterization. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 12048-12052. (7) Simonin, F.; Befort, K.; Gaveriaux-Ruff, C.; Matthes, H.; Nappey, V.; Lannes, B.; Micheletti, G.; Kieffer, B. The Human δ-Opioid Receptor: Genomic Organization, cDNA Cloning, Functional Expression, and Distribution in Human Brain. Mol. Pharmacol. 1994, 46, 1015-1021.

2880


Ananthan et al.

(8) Yasuda, K.; Raynor, K.; Kong, H.; Breder, C. D.; Takeda, J.; Reisine, T.; Bell, G. I. Cloning and Functional Comparison of κ and δ Opioid Receptors from Mouse Brain. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 6736-6740. (9) Ni, Q.; Xu, H.; Partilla, J. S.; Calderon, S. N.; Porreca, F.; Rice, K. C.; Bertha, C. M.; McNutt, R. W.; Ananthan, S.; Rothman, R. B. Opioid Peptide Receptor Studies. 2. [3H]SNC-121, A Novel, Selective, and High-Affinity Ligand for Opioid Delta Receptors. Analgesia 1995, 1, 185-193. (10) Ohkawa, S.; DiGiacomo, B.; Larson, D. L.; Takemori, A. E.; Portoghese, P. S. 7-Spiroindanyl Derivatives of Naltrexone and Oxymorphone as Selective Ligands for δ Opioid Receptors. J. Med. Chem. 1997, 40, 1720-1725. (11) Palmer, R. B.; Upthagrove, A. L.; Nelson, W. L. (E)- and (Z)-7-Arylidenenaltrexones: Synthesis and Opioid Receptor Radioligand Displacement Assays. J. Med. Chem. 1997, 40, 749753. (12) Calderon, S. N.; Rothman, R. B.; Porreca, F.; Flippen-Anderson, J. L.; McNutt, R. W.; Xu, H.; Smith, L. E.; Bilsky, E. J.; Davis, P.; Rice, K. C.; Probes for Narcotic Receptor Mediated Phenomena. 19. Synthesis of (+)-4-[(RR)-R-((2S,5R)-4-Allyl-2,5-dimethyl1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80): A Highly Selective, Nonpeptide δ Opioid Receptor Agonist. J. Med. Chem. 1994, 37, 2125-2128. (13) Calderon, S. N.; Rice, K. C.; Rothman, R. B.; Porreca, F.; FlippenAnderson, J. L.; Kayakiri, H.; Xu, H.; Becketts, K.; Smith, L. E.; Bilsky, E. J.; Davis, P.; Horvath, R. Probes for Narcotic Receptor Mediated Phenomena. 23. Synthesis, Opioid Receptor Binding and Bioassay of the Highly Selective δ Agonist (+)-4[(RR)-R-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80) and Related Novel Nonpeptide δ Opioid Receptor Ligands. J. Med. Chem. 1997, 40, 695-704. (14) Suzuki, T.; Tsuji, M.; Mori, T.; Misawa, M.; Endoh, T.; Nagase, H. Effects of a Highly Selective Nonpeptide δ Opioid Receptor Agonist, TAN-67, on Morphine-Induced Antinociception in Mice. Life Sci. 1995, 57, 155-168. (15) Bilsky, E. J.; Calderon, S. N.; Wang, T.; Bernstein, R. N.; Davis, P.; Hruby, V. J.; McNutt, R. W.; Rothman, R. B.; Rice, K. C.; Porreca, F. SNC 80, A Selective, Nonpeptidic and Systemically Active Opioid Delta agonist. J. Pharmacol. Exp. Ther. 1995, 273, 359-366 and references therein. (16) Abdelhamid, E. E.; Sultana, M.; Portoghese, P. S.; Takemori, A. E. Selective Blockage of Delta Opioid Receptors Prevents the Development of Morphine Tolerance and Dependence in Mice. J. Pharmacol. Exp. Ther. 1991, 258, 299-303. (17) Menkens, K.; Bilsky, E. J.; Wild, K. D.; Portoghese, P. S.; Reid, L. D.; Porreca, F. Cocaine Place Preference is Blocked by the δ-Opioid Receptor Antagonist, Naltrindole. Eur. J. Pharmacol. 1992, 219, 345-346. (18) Reid, L. D.; Hubbell, C. L.; Glaccum, M. B.; Bilsky, E. J.; Portoghese, P. S.; Porreca, F. Naltrindole, An Opioid Delta Receptor Antagonist, Blocks Cocaine-induced Facilitation of Responding for Rewarding Brain Stimulation. Life Sci. 1993, 52, PL67-PL71. (19) Arakawa, K.; Akami, T.; Okamoto, M.; Akioka, K.; Nakai, I.; Oka, T.; Nagase, H. Immunosuppression by Delta Opioid Receptor Antagonist. Transplant. Proc. 1993, 25, 738-740. (20) House, R. V.; Thomas, P. T.; Kozak, J. T.; Bhargava, H. N. Suppression of Immune Function by Non-peptidic Delta Opioid Receptor Antagonists. Neurosci. Lett. 1995, 198, 119-122. (21) Mattia, A.; Vanderah, T.; Mosberg, H. I.; Porreca, F. Lack of Antinociceptive Cross-Tolerance between [D-Pen2, D-Pen5]Enkephalin and [D-Ala2]Deltorphin II in Mice: Evidence for Delta Receptor Subtypes. J. Pharmacol. Exp. Ther. 1991, 258, 583-587. (22) Sofuoglu, M.; Portoghese, P. S.; Takemori, A. E. Differential Antagonism of Delta Opioid Agonists by Naltrindole and its Benzofuran Analog (NTB) in Mice: Evidence for Delta Opioid Receptor Subtypes. J. Pharmacol. Exp. Ther. 1991, 257, 676680. (23) Jiang, Q.; Takemori, A. E.; Sultana, M.; Portoghese, P. S.; Bowen, W. D.; Mosberg, H. I.; Porreca, F. Differential Antagonism of Opioid Delta Antinociception by [D-Ala2, Leu5, Cys6]Enkephalin and Naltrindole 5′-Isothiocyanate: Evidence for Delta Receptor Subtypes. J. Pharmacol. Exp. Ther. 1991, 257, 1069-1075. (24) Fang, L.; Knapp, R. J.; Horvath, R.; Matsunaga, T. O.; Haaseth, R. C.; Hruby, V. J.; Porreca, F.; Yamamura, H. I. Characterization of [3H]Naltrindole Binding to Delta Opioid Receptors in Mouse Brain and Mouse Vas Deferens: Evidence for Delta Opioid Receptor Heterogeneity. J. Pharmacol. Exp. Ther. 1994, 268, 836-846. (25) Lai, J.; Bilsky, E. J.; Rothman, R. B.; Porreca, F. Treatment with Antisense Oligodeoxynucleotide to the Opioid Delta Receptor Selectively Inhibits Delta-2 Agonist Antinociception. Neuroreport 1994, 5, 1049-1052.

(26) Schoffelmeer, A. N. M.; Yao, Y. H.; Gioannini, T. L.; Hiller, J. M.; Ofri, D.; Roques, B. P.; Simon, E. J. Cross-Linking of Human [125I]β-Endorphin to Opioid Receptors in Rat Striatal Membranes: Biochemical Evidence for the Existence of a mu/delta Opioid Receptor Complex. J. Pharmacol. Exp. Ther. 1990, 253, 419-426. (27) Rothman, R. B.; Holaday, J. W.; Porreca, F. Allosteric Coupling Among Opioid Receptors: Evidence for an Opioid Receptor Complex. In Handbook of Experimental Pharmacology, Volume 104, Opioids I; Herz, A., Akil, H., Simon, E. J., Eds; SpringerVerlag: Berlin, 1993; pp 217-237. (28) Cha, X. Y.; Xu, H.; Rice, K. C.; Porreca, F.; Lai, J.; Ananthan, S.; Rothman, R. B. Opioid Peptide Receptor Studies. 1. Identification of a Novel δ-Opioid Receptor Binding Site in Rat Brain Membrane. Peptides 1995, 16, 191-198. (29) Xu, H.; Partilla, J. S.; de Costa, B. R.; Rice, K. C.; Rothman, R. B. Differential Binding of Opioid Peptides and Other Drugs to Two Subtypes of Opioid Delta ncx Binding Sites in Mouse Brain: Further Evidence for Delta Receptor Heterogeneity. Peptides 1993, 14, 893-907. (30) Raynor, K.; Kong, H.; Chen, Y.; Yasuda, K.; Yu, L.; Bell, G. I.; Reisine, T. Pharmacological Characterization of the Cloned κ-, δ-, and µ-Opioid Receptors. Mol. Pharmacol. 1994, 45, 330-334. (31) Wang, H. Q.; Kampine, J. P.; Tseng, L. F. Antisense Oligodeoxynucleotide to a δ-Opioid Receptor Messenger RNA Selectively Blocks the Antinociception Induced by Intracerebroventricularly Administered δ-, but not µ-, - or κ-Opioid Receptor Agonists in the Mouse. Neuroscience 1996, 75, 445-452. (32) Cha, X. Y.; Xu, H.; Ni, Q.; Partilla, J. S.; Rice, K. C.; Matecka, D.; Calderon, S. N.; Porreca, F.; Lai, J.; Rothman, R. B. Opioid Peptide Receptor Studies. 4. Antisense Oligodeoxynucleotide to the Delta Opioid Receptor Delineates Opioid Receptor Subtypes. Regul. Pept. 1995, 59, 247-253. (33) Portoghese, P. S.; Sultana, M.; Nagase, H.; Takemori, A. E. Application of the Message-Address Concept in the Design of Highly Potent and Selective Non-Peptide δ-Opioid Receptor Antagonists. J. Med. Chem. 1988, 31, 281-282. (34) Portoghese, P. S.; Sultana, M.; Takemori, A. E. Design of Peptidomimetic δ Opioid Receptor Antagonists Using the Message-Address Concept. J. Med. Chem. 1990, 33, 1714-1720. (35) Takemori, A. E.; Portoghese, P. S. Selective Naltrexone-Derived Opioid Receptor Antagonists. Annu. Rev. Pharmacol. Toxicol. 1992, 32, 239-269. (36) Portoghese, P. S.; Sultana, M.; Takemori, A. E. Naltrindole 5′-Isothiocyanate: A Nonequilibrium, Highly Selective δ Opioid Receptor Antagonist. J. Med. Chem. 1990, 33, 1547-1548. (37) Portoghese, P. S.; Sultana, M.; Nelson, W. L.; Klein, P.; Takemori, A. E. δ Opioid Antagonist Activity and Binding Studies of Regioisomeric Isothiocyante Derivatives of Naltrindole: Evidence for δ Receptor Subtypes. J. Med. Chem. 1992, 35, 40864091. (38) Portoghese, P. S.; Nagase, H.; MaloneyHuss, K. E.; Lin, C.-E.; Takemori, A. E. Role of Spacer and Address Components in Peptidomimetic δ Opioid Receptor Antagonists Related to Naltrindole. J. Med. Chem. 1991, 34, 1715-1720. (39) Portoghese, P. S.; Larson, D. L.; Sultana, M.; Takemori, A. E. Opioid Agonist and Antagonist Activities of Morphindoles Related to Naltrindole. J. Med. Chem. 1992, 35, 4325-4329. (40) Olmsted, S. L.; Takemori, A. E.; Portoghese, P. S. A Remarkable Change of Opioid Receptor Selectivity on the Attachment of a Peptidomimetic κ Address Element to the δ Antagonist, Naltrindole: 5′-[N2-Alkylamidino)methyl]naltrindole Derivatives as a Novel Class of κ Opioid Receptor Antagonists. J. Med. Chem. 1993, 36, 179-180. (41) Korlipara, V. L.; Takemori, A. E.; Portoghese, P. S. N-Benzylnaltrindoles as Long-Acting δ-Opioid Receptor Antagonists. J. Med. Chem. 1994, 37, 1882-1885. (42) Portoghese, P. S.; Ohkawa, S.; Moe, S. T.; Takemori, A. E. Synthesis and δ-Opioid Receptor Antagonist Activity of a Naltrindole Analogue with a Regioisomeric Indole Moiety. J. Med. Chem. 1994, 37, 1886-1888. (43) Korlipara, V. L.; Takemori, A. E.; Portoghese, P. S. Electrophilic N-Benzylnaltrindoles as δ Opioid Receptor-Selective Antagonists. J. Med. Chem. 1995, 38, 1337-1343. (44) Schmidhammer, H.; Daurer D.; Wieser, M. Synthesis and Biological Evaluation of 14-Alkoxymorphinans. 14. 14-Ethoxy5-Methyl Substituted Indolomorphinans with δ Opioid Receptor Selectivity. Bioorg. Med. Chem. Lett. 1997, 7, 151-156. (45) Portoghese, P. S. The Role of Concepts in Structure-Activity Relationship Studies of Opioid Ligands. J. Med. Chem. 1992, 35, 1927-1937. (46) Niwa, H. Biphenyl Derivatives. IV. Dialkylaminoacetylhydrazinobiphenyl Derivatives. Tohoku Yakka Daigaku Kiyo 1957, 4, 61-68. Chem. Abstr. 1958, 52, 7236.

Indolic Benzene Substituted Naltrindole Analogues (47) Hunsberger, I. M.; Shaw, E. R.; Fugger, J.; Ketcham, R.; Lednicer, D. The Preparation of Substituted Hydrazines. IV. Arylhydrazines via Conventional Methods. J. Org. Chem. 1956, 21, 394-399. (48) Hahn, V.; Skaric, V.; Miler, E.; Stojanac, Z. Syntheses in the Diphenyl Ether Series. IV. The Phenoxyphenylhydrazines. Croat. Chem. Acta 1956, 28, 57-65. (49) Keglevic, D.; Stojanac, N.; Desaty, D. The Synthesis of 3,5Disubstituted Indoles by Cyclization under Mild Conditions. Croat. Chem. Acta 1961, 33, 83-88. (50) Mentzer, C.; Beaudet, C.; Bory, M. Indoles VII. Application of a New Cyclization Technique to the Synthesis of 5-Benzyloxy-3indoleacetic Acid. Bull. Soc. Chim. Fr. 1953, 421-423. (51) Smissman, E. E.; Corbett, M. D. The Syntheses of 4-Acylamido1,4-benzoxazine-2,3-diones and 4-(p-Toluenesulfonamido)-1,4benzoxazine-2,3-dione. J. Org. Chem. 1972, 37, 1704-1707. (52) Rothman, R. B.; Xu, H.; Seggel, M.; Jacobson, A. E.; Rice, K. C.; Brine, G. A.; Carroll, F. I. RTI-4614-4: An Analog of (+)-cis3-Methylfentanyl with a 27, 000-fold Binding Selectivity for Mu Versus Delta Opioid Binding Sites. Life Sci. 1991, 48, PL111PL116. (53) Rothman, R. B.; Bykov, V.; Ofri, D.; Rice, K. C. LY164929: A Highly Selective Ligand for the Lower Affinity [3H]D-Ala2-DLeu5-Enkephalin Binding Sites. Neuropeptides 1988, 11, 1316. (54) Rothman, R. B.; Bykov, V.; de Costa, B. R.; Jacobson, A. E.; Rice, K. C.; Brady, L. S. Interaction of Endogenous Opioid Peptides and Other Drugs with Four Kappa Opioid Binding Sites in Guinea Pig Brain. Peptides 1990, 11, 311-331. (55) Ni, Q.; Xu, H.; Partilla, J. S.; de Costa, B. R.; Rice, K. C.; Kayakiri, H.; Rothman, R. B. Opioid Peptide Receptor Studies. 3. Interaction of Opioid Peptides and Other Drugs With Four Subtypes of the k2 Receptor in Guinea Pig Brain. Peptides 1995, 16, 1083-1095.


2881

(56) Portoghese, P. S.; Lin, C.-E.; Farouz-Grant, F.; Takemori, A. E. Structure-Activity Relationship of N17′-Substituted Norbinaltorphimine Congeners. Role of N17′ Basic Group in the Interaction with a Putative Address Subsite on the κ Opioid Receptor. J. Med. Chem. 1994, 37, 1495-1500. (57) Hjorth, S. A.; Thirstrup, K.; Grandy, D. K.; Schwartz, T. W. Analysis of Selective Binding Epitopes for the κ-Opioid Receptor Antagonist Nor-binaltorphimine. Mol. Pharmacol. 1995, 47, 1089-1094. (58) Kramer, T. H.; Davis, P.; Hruby, V. J.; Burks, T. F.; Porreca, F. In vitro Potency, Affinity and Agonist Efficacy of Highly Selective Delta Opioid Receptor Ligands. J. Pharmacol. Exp. Ther. 1993, 266, 577-584. (59) Fundytus, M. E.; Schiller, P. W.; Shapiro, M.; Weltrowska, G.; Coderre, T. J. Attenuation of Morphine Tolerance and Dependence with the Highly Selective δ-Opioid Receptor Antagonist TIPP[ψ]. Eur. J. Pharmacol. 1995, 286, 105-108. (60) Schmidt, R.; Vogel, D.; Mrestani-Klaus, C.; Brandt, W.; Neubert, K.; Chung, N. N.; Lemieux, C.; Schiller, P. W. Cyclic β-Casomorphin Analogues with Mixed µ Agonist/δ Antagonist Properties: Synthesis, Pharmacological Characterization, and Conformational Aspects. J. Med. Chem. 1994, 37, 1136-1144. (61) Schmidt, R.; Wilkes, B. C.; Chung, N. N.; Lemieux, C.; Schiller, P. W. Effect of Aromatic Amino Acid Substitutions in the 3-position of Cyclic β-Casomorphin Analogues on µ-Opioid Agonist/δ-Opioid Antagonist Properties. Int. J. Pept. Protein Res. 1996, 48, 411-419. (62) Porreca, F.; LoPresti, D.; Ward, S. J. Opioid Agonist Affinity in the Guinea-pig Ileum and Mouse Vas Deferens. Eur. J. Pharmacol. 1990, 179, 129-139. (63) MINSQ Least Squares Parameter Estimation, version 3.05; MicroMath, Inc., 1989.

JM980083I

Synthesis, Opioid Receptor Binding, and Bioassay of Naltrindole

Recommend Documents